The invention relates to a transconductance amplifier comprising two transistors T.sub.o each having an emitter area e.sub.o, their bases constituting inputs for receiving an input voltage and their emitters being connected to a current source, and further comprising at least a first output for an output current.
Such transconductance amplifiers, briefly referred to as transconductors, are suitable for general uses and in particular for use in filter circuits, multipliers and oscillators.
A transconductor is a voltage-controlled current source in which the proportionality factor between the output current and the input voltage is given by the transconductance. The simplest transconductor is a differential amplifier, by means of which a voltage applied between the bases is converted into two collector signal currents of opposite phase. In a differential amplifier these signal currents increase as a linear function of the input voltage over a small range only, so that the transconductance is only constant over a very small range of input voltage. The article "Bipolar Integration of analog gyrator and laguerre type filters (transconductor-capacitor filters)" in Proceedings ECCTD'83, September 1983, pages 107-110 describes a linearised transconductor which comprises two parallel-connected differential amplifiers, the transistors of each amplifier having different emitter areas and the bases and the collectors of two transistors having different emitter areas being interconnected. In the case of a suitable choice of the ratio between the emitter areas of the transistors, the linear range of this transconductor is approximately five times as large as that of a single differential amplifier. In addition, this known transconductor may be arranged to form a square-law transconductor, in which the output current increases as a square-law function of the input voltage over a specific range. In this case the collectors of the transistors whose bases are interconnected are not coupled to each other but are cross-coupled to the collector of the corresponding transistor of the other differential amplifier.